Found during testing of MDL-66667. The generated course images are ~7kb inline svg's. These never change for a given course and are deterministic, they are a good candidate to a plugin file which generates the file on demand and serves is (ie does not store the file in File API) and then mark this as both public and immutable so its cached in cdn's.

The plugin file should only need to contain seed id and no other identifiers so can be safely public.

When this is in place the data source in MDL-66667 should be updated to use it.